<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>翻转菜单</title>
    <style>
    *{
        margin: 0;
        padding: 0;
    }

    li{
        list-style: none;
        text-align: center;
    }
    
    .nav{
        margin: 100px auto;
        width: 300px;
        height: 40px;
        line-height: 40px;
        background-color:skyblue;
    }

    .nav>li{
        float: left;
        width: 100px;
        height: 40px;
    }

    .menus li{
        background-color: rgb(4, 245, 193);
        transform: rotateY(180deg);
        transition: all 1s;
        opacity: 0;
    }
    .nav>li:hover .menus>li{
        transform: none;
        opacity: 1;
    }

    .nav>li:hover .menus li:nth-child(1){
        transition-delay: 0ms;
    }
    .nav>li:hover .menus li:nth-child(2){
        transition-delay: 200ms;
    }
    .nav>li:hover .menus li:nth-child(3){
        transition-delay: 400ms;
    }
    .nav>li:hover .menus li:nth-child(4){
        transition-delay: 600ms;
    }

    .nav>li .menus>li:nth-child(1){
        transition-delay: 600ms;
    }
    .nav>li .menus>li:nth-child(2){
        transition-delay: 400ms;
        
    }
    .nav>li .menus>li:nth-child(3){
        transition-delay: 200ms;
        
    }
    .nav>li .menus>li:nth-child(4){
        transition-delay: 0ms;
    }
    </style>
</head>
<body>
    <ul class="nav">
        <li>分类1
            <ul class="menus">
                <li>子菜单</li>
                <li>子菜单</li>
                <li>子菜单</li>
                <li>子菜单</li>
            </ul>
        </li>
        <li>分类2
            <ul class="menus">
                <li>子菜单</li>
                <li>子菜单</li>
                <li>子菜单</li>
                <li>子菜单</li>
            </ul>
        </li>
        <li>分类3
            <ul class="menus">
                <li>子菜单</li>
                <li>子菜单</li>
                <li>子菜单</li>
                <li>子菜单</li>
            </ul>
        </li>
    </ul>

</body>
</html>